Methods of lyophilizing or "freeze-drying" viruses, biologically active molecules, and bacteria are routinely used in laboratories. Typically, the cells or molecules to be lyophilized are suspended in a solution that allows recovery of the desired activity after the freezing process. Ideal formulations for lyophilization solutions should provide a stabilizing environment for a finite time before lyophilization; provide good thermal and freezing properties during lyophilization; and cryoprotect the desired activity.
Previous methods of lyophilization have focused primarily on using sugars in preservation of proteins in eukaryotic cells and preservation of bacteria. For example, the primary ingredient of solutions for lyophilizing bacterial cells is a sugar at a concentration of 5-10%, used to stabilize the cells (Greaves, Fundamental Aspects of Freeze-drying Bacterial and Living Cells, in Aspects Theoriques et Industriels de la Lyophilisation, 407-410 (Rey ed. 1964). However, effective eukaryotic cell lyophilization methods for preserving nucleic acids, particularly RNA, have not been established. The inclusion of sugar such as lactose in an isotonic eukaryotic cell lyophilization formulation did not give high yields of nucleic acid isolated from lyophilized cells. The inclusion of preserving materials, such as low concentrations of methanol or ethanol in the lyophilization solutions was also found to be ineffective for isolation of intact nucleic acid after lyophilization.
Methods of long term preservation of nucleic acids from eukaryotic cells are desirable due to the use of molecular biological approaches for disease diagnosis and forensics. In such assays, positive and negative controls from a reference source are particularly useful. Clinical samples from previous patients and cell lines with known genotypes are often used as sources of experimental control cells. However, clinical samples from patients are typically available in limited amounts. Cultured cell lines are convenient sources of control cells, however, the use of freshly cultured cells for each assay is problematic because of the time, cost, and labor needed for growth and maintenance of cultured cells.